Using a “T” connection is not a problem, I visited our local automotive and hydraulic parts dealer and purchased a couple of “T” connectors, plus hose tails plus stop end connections when I wanted to connect in a fuel polishing system last winter. This enabled me to connect in and use a “Patrick Laine” style fuel polisher While leaving the fuel system otherwise intact. (Thanks Patrick, and I hope I haven’t spelled your name wrong). By having the “T” pieces in place it meant that in addition to fuel polishing, I could also run the diesel heater, and all while staying onboard my boat including overnight during last winter. Okay on this occasion it was the polishing system that was connected via the “T” piece connections, while the heater remained connected as originally set up via second outlet on the primary fuel filter, but it could just as easily have been the diesel heater that had been connected through a “T” piece. Once the fuel polishing had been done, the polishing system was removed and the now unused “T” piece connections were then blanked off using the stop end connections purchased earlier.
As for fuel leaks, “wot leaks,” there weren’t any.
The “T” pieces remain connected with one end temporarily blanked off but otherwise ready for use again next winter when the tank contents will be polished again, and again every winter all the while I keep my boat.